ABSTRACT

ObjectiveTo determine the US national prevalence of attention-deficit/hyperactivity disorder (ADHD) and whether prevalence, recognition, and treatment vary by socioeconomic group.

DesignCross-sectional survey.

SettingNationally representative sample of the US population from 2001 to 2004.

ParticipantsEight- to 15-year-old children (N = 3082) in the National Health and Nutrition Examination Survey.

Main Outcome MeasuresThe Diagnostic Interview Schedule for Children (caregiver module) was used to ascertain the presence of ADHD in the past year based on Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) (DSM-IV) criteria. Prior diagnosis of ADHD by a health professional and ADHD medication use were assessed by caregiver report.

ResultsOf the children, 8.7% met DSM-IV criteria for ADHD. The poorest children (lowest quintile) were more likely than the wealthiest (highest quintile) to fulfill criteria for ADHD (adjusted odds ratio [AOR], 2.3; 95% confidence interval [CI], 1.4-3.9). Among children meeting DSM-IV ADHD criteria, 47.9% had a prior diagnosis of ADHD and 32.0% were treated consistently with ADHD medications during the past year. Girls were less likely than boys to have their disorder identified (AOR, 0.3; 95% CI, 0.1-0.8), and the wealthiest children were more likely than the poorest to receive regular medication treatment (AOR, 3.4; 95% CI, 1.3-9.1).

ConclusionsOf US children aged 8 to 15 years, 8.7%, an estimated 2.4 million, meet DSM-IV criteria for ADHD. Less than half of children meeting DSM-IV criteria report receiving either a diagnosis of ADHD or regular medication treatment. Poor children are most likely to meet criteria for ADHD yet are least likely to receive consistent pharmacotherapy.

Despite widespread concern that the rate of attention-deficit/hyperactivity disorder (ADHD) is on the rise,1 the national population-based prevalence of ADHD in US children has not been firmly established. Prior national population-based studies of American children have primarily used parental report of an ADHD diagnosis, medical record review, or medication use2- 9 as proxies for a formal ADHD diagnostic assessment. Other studies have applied Diagnostic and Statistical Manual of Mental Disorders (Third Edition)10(DSM-III), Diagnostic and Statistical Manual of Mental Disorders (Third Edition–Revised)11 (DSM-III-R), and Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition)12 (DSM-IV) criteria to determine diagnostic status, but those samples have been regional or multisite cohort studies.13- 30 Owing to differing methods of determining diagnostic status and regional variations in prevalence, estimates of ADHD prevalence in community-based American samples have ranged from 2% to 26%.13- 30 To our knowledge, to date, no US nationally representative population-based studies using DSM-IV criteria for ADHD have been conducted in children.

In addition to ambiguity regarding overall ADHD prevalence, there is uncertainty about prevalence of the ADHD subtypes. The DSM-IV ADHD subtypes are useful because each is differentially associated with delinquency, comorbid conditions, and life outcomes.31 Most of what is known about ADHD subtype prevalence comes from clinic-based samples, which have demonstrated higher rates of ADHD–combined type (ADHD-CT) than the other subtypes.31,32 However, international and US regional population-based studies have had mixed results, with some showing a predominance of the inattentive subtype (ADHD-IA)16,27,33 and others documenting highest rates of ADHD-CT.30,34,35

The extent to which the prevalence of ADHD and its subtypes varies by population characteristics, in particular socioeconomic and racial/ethnic status, is also unclear. Although an elevated ADHD prevalence is plausible in lower-income populations because of their higher rates of putative ADHD risk factors such as low birth weight,36 lead exposure,37 and in utero tobacco exposure,38 national studies have not observed a difference in rates by income9 or have identified income-related differences among boys but not girls.5 However, because these studies relied on caregiver report of an ADHD diagnosis, the findings may reflect differential access to health care or diagnostic bias. Results from US regional studies using DSM-III, DSM-III-R, and DSM-IV criteria to study ADHD rates in different income groups have varied, with 2 documenting an elevated ADHD prevalence in lower-income children and 1 finding no difference.21,24,25 Studies of ADHD rates in US racial/ethnic groups have also been inconsistent. The US national studies have shown lower rates of reported ADHD diagnoses in children from minority backgrounds,7,9 but 2 regional studies found no difference in DSM-based ADHD rates in non-Hispanic white and African American children.21,30 Little information is available about DSM-based rates of ADHD in Mexican American children,27,39 one of the fasting growing sectors of the US population.

There is also broad public concern about rates of psychostimulant use for ADHD,40 yet these concerns are difficult to evaluate because most studies have not benchmarked the appropriateness of medication treatment against actual DSM-based diagnostic status.15,28,29 Prior national studies have documented lower rates of ADHD medication use in uninsured children, African American children, and Hispanic children but no difference in use by income.3,5,41 However, because these studies examined psychostimulant use without information about ADHD symptoms and impairment, it is unclear whether differences in medication use signify undertreatment in specific groups.

The purpose of this study was to examine ADHD prevalence in a national population-based sample of US children assessed with a DSM-IV–based diagnostic instrument. We also investigated sociodemographic predictors of ADHD and the likelihood of receiving a prior diagnosis and medication treatment among children who fulfill DSM-IV ADHD criteria.

METHODS

SAMPLE

The National Health and Nutrition Examination Survey (NHANES)42 is an annual multistage probability sample survey of the noninstitutionalized US population, including an oversample of minority populations. A total of 3907 children aged 8 to 15 years participated in NHANES from 2001 to 2004, with data regarding DSM-IV ADHD diagnostic status available for 3082 children (78.9% of total). Approximately 10% of children lacked DSM-IV ADHD data because they did not complete a previously required survey component at the NHANES mobile examination center, and caregivers of another 10% could not be located for the ADHD interview, refused, or had a primary language other than English or Spanish. Those with DSM-IV ADHD information did not differ from those without in terms of sex (P = .84), report of prior ADHD diagnosis (P = .93), or health insurance status (P = .45). However, those lacking DSM-IV ADHD data were significantly more likely to be younger (mean age, 9.9 years vs 12.1 years), poorer (lowest income quintile, 24.9% vs 18.9%), and African American (17.0% vs 14.7%).

OUTCOMES

The National Institute of Mental Health Diagnostic Interview Schedule for Children–IV (DISC-IV), a structured diagnostic interview instrument designed for use in epidemiologic studies, was used to assess the presence of ADHD based on DSM-IV criteria. This study used the ADHD DISC caregiver module, which has evidence of substantial validity,43 reliability for both its English43 and Spanish44,45 versions, and successful use with the telephone in the DSM-IV field trials.32 Caregivers completed the DISC telephone interview 2 to 4 weeks after the child's NHANES mobile examination center evaluation, providing information about the child's ADHD symptoms, age at onset, symptom pervasiveness, and related impairments during the prior 12 months. The DISC algorithms determine ADHD diagnostic status and ascertain ADHD subtype (ADHD-IA, ADHD-CT, and hyperactive-impulsive type [ADHD-HI]). In addition, questions on the DISC inquire about use of “medicine for being overactive, being hyperactive, or having trouble paying attention” in the past 12 months and whether medication was taken “most of the time during the last year” (ie, consistent medication treatment) (D. J. Brody, MPH, written communications, October 17, 2006). To determine whether a child had been previously diagnosed as having ADHD, caregivers were asked, “Has a doctor or health professional ever told you that [child's name] had attention deficit disorder?”46 during the administration of a separate NHANES interview module.

PREDICTORS

Predictors considered in these analyses included the child's age, sex, race/ethnicity, household income to poverty line ratio (PIR), and health insurance status. The PIR is the ratio of the reported household income to the poverty threshold appropriate for household size. To capture the distribution across the US population, the PIR was categorized into quintiles. Child race/ethnicity was designated by caregivers and included the following categories: non-Hispanic black, Mexican American, other Hispanic, non-Hispanic white, and other (including multiracial). Because of the small number of subjects in the “other Hispanic” and “other (including multiracial)” groups (n = 115 and n = 107, respectively), they were combined into a single “other race/ethnicity” category.

ANALYSES

To account for the complex survey design, sample weights and design variables were applied according to National Center for Health Statistics guidelines to generate all estimates. These sample weights were calculated from the base probabilities of selection, adjusted for nonresponse, and poststratified to match population control totals. Analyses were performed using SAS statistical software, version 9 (SAS Institute Inc, Cary, North Carolina), procedures for analysis of complex surveys.

Descriptive statistics on the national prevalence of ADHD and ADHD subtypes are given for the sample overall and across socioeconomic and racial/ethnic groups. For children meeting DSM-IV ADHD criteria, rates of caregiver-reported diagnoses (ie, prior disorder recognition) and medication treatment are also summarized overall and across demographic groupings. We used χ2 tests to assess bivariate associations between the demographic predictor variables and ADHD diagnostic status, prior disorder recognition, and treatment status. To adjust for the effects of the demographic variables on ADHD status, prior disorder recognition, and history of ADHD treatment, multivariable logistic regression was performed with income, race/ethnicity, age, and sex included as predictors. In addition, for models predicting prior disorder recognition and history of ADHD treatment, health insurance status and ADHD subtype were included to account for differential access to care and the potential bias toward diagnosing and treating children with more externalizing behaviors.

The institutional review board of Cincinnati Children's Hospital Medical Center determined this study to be exempt from its review.

RESULTS

PREVALENCE OF ADHD

Among participants aged 8 to 15 years, 8.7% (95% confidence interval [CI], 7.3%-10.1%) met DSM-IV criteria for ADHD in the year prior to the survey, equivalent to 2.4 million children in the United States (Table 1). An additional 3.3% (95% CI, 2.4%-4.1%) did not meet DSM-IV ADHD criteria but had both a parent-reported prior diagnosis of ADHD and treatment with ADHD medications at some time during the past 12 months. However, the following results focus on those meeting DSM-IV ADHD criteria in the past year.

In bivariate analyses, rates of meeting DSM-IV ADHD criteria were higher in boys than girls (11.8% vs 5.4%; P < .001) and higher in non-Hispanic white children than Mexican Americans or children of “other race/ethnicity” (Table 2).

PREVALENCE OF ADHD SUBTYPES

Of the children, 4.4% met criteria for ADHD-IA, 2.2% for ADHD-CT, and 2.0% for ADHD-HI (Table 1). Subtype differences were observed among the various income and racial/ethnic groups (Table 3). The poorest children had a higher likelihood of ADHD-HI compared with the wealthiest (AOR for PIR, first vs fifth quintile, 3.1; 95% CI, 1.2-8.3). African Americans and Mexican Americans had a lower likelihood of ADHD-IA rates compared with non-Hispanic white children (AOR, 0.4; 95% CI, 0.2-0.8, for both).

Younger children were observed to have higher ADHD-HI rates compared with older children (2.8% for 8- to 11-year-olds vs 1.3% for 12- to 15-year-olds; P = .03) in bivariate analyses, but this difference reached only borderline significance in multivariable analyses (AOR, 1.9; 95% CI, 0.9-3.9).

MEDICATION USE AMONG CHILDREN MEETING DSM-IV ADHD CRITERIA

Of the children who met DSM-IV ADHD criteria, 38.8% (95% CI, 30.3%-47.3%) reportedly received medication to treat inattention, hyperactivity, or overactivity at any time in the prior year, and 32.0% (95% CI, 25.7%-38.3%) received medication for most of the past year (ie, consistent use). Child age was the only significant sociodemographic predictor of any ADHD medication use in the past year in multivariable analyses, with an increased likelihood of treatment for older children (Table 5). However, income was a significant predictor of consistent medication treatment. Among children meeting DSM-IV ADHD criteria, only 15.5% (95% CI, 5.5%-25.5%) of the poorest children (PIR, first quintile) had received ADHD medications for most of the previous year, with their likelihood of consistent medication receipt being less than one-third that of children in other income groups (Table 5). There was also a trend toward increased regular medication use in older children. Race/ethnicity was not associated with consistent ADHD medication use.

Table Graphic Jump LocationTable 5.Rates and Likelihood of Medication Treatment Among Those With Attention-Deficit/Hyperactivity Disorder (All Types) by DSM-IV Criteria

COMMENT

In a nationally representative sample of children aged 8 to 15 years, 8.7% met DSM-IV criteria for any type of ADHD in the year prior to the survey, equivalent to approximately 2.4 million children. We found a higher prevalence of meeting DSM-IV ADHD criteria in the poorest children, particularly for ADHD-HI. Mexican American children had lower overall rates of ADHD, and both Mexican Americans and African Americans had lower rates of ADHD-IA. Despite rising public awareness of ADHD, less than half of children who met DSM-IV ADHD criteria had reportedly had their conditions diagnosed by a health care professional or been treated with medications. Girls were less likely to have their disorder recognized, and the poorest children were least likely to receive consistent ADHD medication treatment.

ADHD AND ADHD SUBTYPE PREVALENCE

Our estimate of overall ADHD prevalence is within the range of prior ADHD prevalence measurements in US regional studies using DSM-IV criteria (2.6%-11.4%).15,16,20,27,30 However, the present study's use of a national sample allows increased generalizability and more precisely characterizes the magnitude of the disorder. In this sample, ADHD-IA was the most common ADHD subtype, a finding that concurs with 2 US regional and 2 international population-based studies of ADHD prevalence16,27,33,47 but is in contrast to the results of 1 US regional30 and 2 international population-based investigations.34,35 Possible reasons for differences include parents' varying cultural expectations of behavioral norms or changes in media coverage increasing awareness of inattentive symptoms. The distribution of ADHD etiologic factors may also have varied, leading to true differences in subtype predominance.

ADHD PREVALENCE WITHIN SOCIODEMOGRAPHIC SUBGROUPS

The nation's poorest children had an increased likelihood of meeting DSM-IV ADHD criteria. This is consistent with the findings of 2 US regional population-based studies that used DSM-III-R criteria for outcome assessment.21,24 In addition, we found higher rates of ADHD-HI in lower-income children, supporting the results of a prior clinical referral sample in which socioeconomic status was lowest for patients with ADHD-HI and highest for for those with ADHD-IA.31 Because the findings of these DSM-based investigations were in regional, predominately non-Hispanic white communities or clinically referred samples, our nationally representative study improves understanding of the ADHD burden facing poor children across the United States. Reasons for the increased likelihood of ADHD in poorer children may include the elevated prevalence of ADHD risk factors (ie, premature birth36 and in utero38 or childhood exposures to toxic substances37) in this group. In addition, given the high heritability of ADHD48 and its negative impact on social, academic, and career outcomes,49 it is plausible that families with ADHD may cluster within the lower socioeconomic strata.

In accordance with 2 DSM-based investigations, we observed no significant difference in overall ADHD rates between non-Hispanic white children and African Africans,21,30 although African Americans had lower rates of ADHD-IA. We also found that Mexican American children had half the likelihood of meeting DSM-IV criteria for any type of ADHD compared with non-Hispanic white children. Given the paucity of prior data, it is difficult to speculate on reasons for the lower ADHD rate observed in Mexican American children. Our findings may be due to differences in the prevalence of causal risk factors, genetic susceptibility, and/or rates of reporting ADHD symptoms across cultures.29

REPORTED PRIOR DIAGNOSIS RATES AMONG CHILDREN WITH ADHD

We found that 48% of children meeting DSM-IV ADHD criteria had reportedly been diagnosed as having ADHD by a health care professional. Unlike prior national studies, in which African American race was associated with lower rates of reported ADHD diagnosis,7,9 race/ethnicity did not predict prior recognition among those meeting ADHD DSM-IV criteria. In addition, income level and ADHD subtype were not associated with reported prior ADHD diagnosis among children meeting DSM-IV criteria, whereas older age (12-15 years old) and male sex did predict a prior diagnosis. Our results suggest a need for health care professionals to increase their index of suspicion regarding the likelihood of ADHD in girls.50 Despite concerns that ADHD may be overlooked in patients from minority backgrounds51 and those with ADHD-IA,52 we did not find evidence of preferential underrecognition in these groups.

MEDICATION TREATMENT RATES AMONG CHILDREN WITH ADHD

Among children meeting DSM-IV ADHD criteria, 39% had some medication treatment and 32% had consistent medication treatment during the past year. To our knowledge, only a few prior population-based studies, conducted in single-site15,28 or multisite29 samples, have reported on treatment received by children with ADHD using a DSM-diagnosed sample. Rates of ADHD medication treatment among those meeting DSM-IV ADHD criteria were higher in our study than those observed by Jensen et al (12% for use in the previous 12 months)29 and Wolraich et al (15%-27% for current medication use)15 but lower than rates documented by Angold et al (72% for any use during a 4-year period).28 These differences may be due to variations in regional prescribing practices or to differing time frames for outcome assessment.

Although prior national studies found no disparity in medication treatment rates by income,3,41 we found that among children fulfilling DSM-IV ADHD criteria, the poorest children were 3 to 5 times less likely to receive consistent medication treatment compared with other income groups. In a prior regional study using a DSM-III-R–based assessment for ADHD, income-related differences were not observed for the likelihood of any ADHD pharmacotherapy during a multiyear span.28 However, because the prior study did not comment on the duration and regularity of medication treatment by subgroup, a comparison is not available for our findings regarding consistent ADHD medication treatment. Although prior national studies found that racial/ethnic minority populations are less likely to use ADHD medications compared with non-Hispanic white children,3,5 we found no significant racial/ethnic group differences in the likelihood of pharmacotherapy among those with DSM-IV–defined ADHD.

LIMITATIONS

One limitation relates to how the ADHD diagnosis was derived for this study. The American Academy of Pediatrics recommends that both caregiver and teacher reports be used to inform a clinical diagnostic evaluation for ADHD.52 However, DSM-IV criteria state simply that impairment must exist in 2 or more settings and do not specifically require 2 reporters.12 The NHANES used only the DISC caregiver module for ADHD assessment, a structured diagnostic interview designed for use in large-scale epidemiologic surveys.43 Notably, the DISC interview assesses symptom pervasiveness by asking caregivers to report symptoms and impairment at home and at school or other activities.12 To our knowledge, no prior US population-based studies using DSM-IV ADHD criteria have gathered clinical data from both caregivers and teachers.15,16,20,27,30 Nonetheless, lower overall rates of ADHD and different subtype rates may result when reports from both teachers and parents are available.53,54

Another potential limitation pertains to the influence of cultural behavioral norms on caregiver symptom reports, as sociodemographic differences in prevalence may be partially due to culturally divergent ratings of ADHD symptoms.55 In addition, circumstances are difficult to interpret for the 3.3% of children who had a reported diagnosis of ADHD and were receiving medication but did not meet DSM-IV ADHD criteria. The NHANES’ cross-sectional design does not allow us to distinguish those who intially met DSM-IV criteria but whose symptoms were reduced below the threshold by medication from those who were prescribed medication inappropriately. Therefore, these 3.3% were not included in our overall ADHD prevalence measurement, a factor that may make our estimate of 8.7% a conservative approximation. Missing data could also have altered the observed prevalence rates. However, given that the analyses incorporated sample weights accounting for differential nonresponse, bias due to this factor is minimized. Furthermore, although this study is of substantial dimension, sample size was still limited for sociodemographic subgroup analyses, particularly for ADHD subtype prevalence.

Finally, our medication treatment outcome was assessed via parent report, whereas review of medical or pharmacy records would have been preferable. In addition, although we are able to determine the prevalence of reported pharmacotherapy in children with DSM-IV–defined ADHD and its variation across sociodemographic groups during the year prior to the survey, we cannot comment on reasons for the rates or differences observed. Although there are often compelling reasons to try stimulant medication treatment in children with ADHD, including robust data confirming short-term symptom improvement, reduced impairment, and protection against future adverse outcomes such as substance abuse,56,57 stimulant medications are sometimes discontinued owing to lack of efficacy or or unacceptable adverse effects.58 Because the NHANES does not contain information about lifetime ADHD medication treatment, we are unable to differentiate children who never tried ADHD medications from those who did not continue because of ineffectiveness or adverse effects. Finally, although behavioral interventions for ADHD are important, the NHANES does not contain specific information about nonpharmacologic treatments for attentional problems.

CONCLUSIONS

We found DSM-IV–diagnosed ADHD to be prevalent in American children and more common among the poorest children. Children from minority backgrounds had lower rates of ADHD-IA, whereas the poorest children had an increased likelihood of ADHD-HI. If this study is replicated, etiologic factors that may partially explain these differences—such as varying rates of in utero tobacco exposure, childhood lead exposure, and complications of pregnancy and delivery—should be investigated so that future public health efforts can be directed at preventing ADHD in groups at highest risk. We also observed that less than half of children who met DSM-IV criteria for ADHD had reportedly had their conditions diagnosed or been treated with ADHD medications, suggesting that some children with clinically significant inattention and hyperactivity may not be receiving optimal interventions. Finally, our finding of a lower likelihood of consistent medication use in the poorest children warrants further investigation and possible intervention to ensure that all children with ADHD have equitable access to treatment when appropriate.

Author Contributions: Dr Froehlich had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: Froehlich, Lanphear, and Kahn. Acquisition of data: Froehlich and Kahn. Analysis and interpretation of data: Froehlich, Lanphear, Epstein, Barbaresi, Katusic, and Kahn. Drafting of the manuscript: Froehlich, Lanphear, and Kahn. Critical revision of the manuscript for important intellectual content: Froehlich, Lanphear, Epstein, Barbaresi, Katusic, and Kahn. Statistical analysis: Froehlich and Kahn. Obtained funding: Froehlich, Lanphear, and Kahn. Administrative, technical, and material support: Froehlich and Kahn. Study supervision: Lanphear, Epstein, and Kahn.

Financial Disclosure: None reported.

Funding/Support: This study was supported by the Ambulatory Pediatrics Association Young Investigator Grant (Dr Froehlich), National Research Service Award grant 1T32PE10027 (Drs Froehlich and Lanphear), grant K23-HD40362-01 from the National Institutes of Health (Dr Kahn), and a Robert Wood Johnson Generalist Physician Faculty Scholars Award (Dr Kahn).

Role of the Sponsors: The funding organizations played no role in the design and conduct of the study, in the collection, management, analysis, and interpretation of the data, or in the preparation, review, or approval of the manuscript.

Independent Statistical Analysis: The NHANES data are collected by the National Center for Health Statistics (NCHS). All analyses, interpretations, and conclusions expressed in this manuscript are those of the authors and not the NCHS, which is responsible only for the initial data.

Additional Contributions: Debra J. Brody, MPH (Division of Health Examination Statistics [NHANES program], National Center for Health Statistics, Centers for Disease Control and Prevention), and Peggy Auinger, MS (Department of Pediatrics, University of Rochester School of Medicine), provided valuable assistance with accessing and analyzing the NHANES dataset. Ms Auinger was compensated for her assistance.

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